Clinical tests of pursuit and optokinetic nystagmus (OKN) are used widely in the diagnosis of opthalmologic and neurologic disorders. However, the clinical value of these tests in localizing lesions in the visual and ocular motor systems has been limited, since our knowledge of the precise pathways subserving pursuit and OKN in man is incomplete. The goal of this project is to evaluate the hypothesis that functionally and anatomically separate pursuit and OKN pathways exist in man. Investigations in lower animals have demonstrated subcortical OKN pathways that utilize the accessory optic system and its projections to the lower brainstem. However, the demonstration of a similar subcortical OKN pathway in man, in addition to the generally-accepted cortical pursuit pathway, requires observations in human subjects. Pursuit and OKN in normal subjects and patients with visual and ocular motor disorders will be induced by carefully controlled stimuli. Eye movements will be recorded with DC electro-oculography and infra-red, scleral reflection techniques, and will be analyzed by a digital computer system. If separate subcortical OKN and cortical pursuit subsystems exist in man, patients with localized lesions of the pursuit pathway should demonstrate characteristics of ocular tracking similar to those in animals that have subcortical OKN and poorly-developed pursuit, such as slow build-up and directional selectivity of OKN. These characteristics will be sought in patients with lesions of the retina, cerebral hemisphere, brainstem and cerebellum. A model of pursuit OKN interaction that assumes separate pursuit and OKN subsystems and linear summation of their outputs when they are stimulated simultaneously will be tested in normal subjects and patients. Corroboration of the model's predictions would be strong, indirect evidence for the existence of separate pathways. The results of this project will be to advance our knowledge of ocular motor pathways in man and to significantly improve the ability of clinical eye movement tests to localize lesions in the visual and ocular motor pathways.